Molded composite mandrel for a downhole zonal isolation tool

ABSTRACT

A composite mandrel includes a filament-wound composite tube, and composite material molded over the filament-wound composite tube. For example, the composite material includes chopped fibers and a matrix of thermoset resin. The chopped fibers are arranged in layers upon the filament-wound composite tube, and the chopped fibers in each of the layers are randomly oriented along first and second orthogonal directions in each of the layers. The composite material includes at least one sheet of the composite material wound over the filament-wound tube, and at least one strip of the composite material wound over the sheet of the composite material and forming a head on the composite mandrel. An internal cavity of the filament-wound composite tube may provide a lumen for the composite mandrel. The internal cavity may be threaded to receive a removable bridge plug.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/194,867 filed Jul. 29, 2011 entitled “Molded Composite Mandrelfor a Downhole Isolation Tool” by James Barlow and Joel Barlow,incorporated herein by reference, which is a divisional of U.S. patentapplication Ser. No. 11/772,804 filed Jul. 2, 2007 entitled “MoldedComposite Mandrel for a Downhole Isolation Tool” by James Barlow andJoel Barlow, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a molded composite mandrel for adownhole zonal isolation tool.

BACKGROUND OF THE INVENTION

A downhole zonal isolation tool often is used for stimulation or servicework upon a well. For example, the zonal isolation tool is a bridgeplug, frac plug, or packer for bridging a hole or gap of a metal tubularsuch as a well casing.

The zonal isolation tool typically has an internal elongated mandrel anda circular array of slips mounted on the mandrel at each end of thetool. Each slip has an outer surface adapted for engagement with theinternal surface of the well casing. Each slip also has an inclinedinner surface. Each array of slips is disposed next to a respectiveconical ring mounted on the mandrel for sliding under the inclined innersurfaces of the slips in the array. In the middle of the zonal isolationtool, rings of elastomeric sealing material are mounted on the mandrelbetween the conical rings.

When a setting tool pulls the mandrel in the longitudinal direction, therings of sealing material expand outward in the radial direction to sealthe well casing. In addition, the conical rings slide under the slipsand force the slips outward in the radial direction into engagement withthe well casing. The slips lock the zonal isolation tool in place insidethe well casing in such a way that the rings of sealing material remainin compression for sealing the well casing when the setting tool isremoved.

The zonal isolation tool can be designed to be retrievable and reusableafter it has been set in the well casing. However, the zonal isolationtool is most economical to manufacture when it has been constructed tobecome permanently set in the well casing so that it must be drilled outdestructively to unseal the well casing.

Traditionally, such a drillable zonal isolation tool has been made of acast iron mandrel and cast iron slips.

A number of downhole tool makers have replaced the cast iron componentsof the zonal isolation tools with composite components of epoxyfiberglass. The composite components can be drilled out faster than castiron, and the drilled-out chips of composite material are lighter thancast iron chips so that the composite chips are more easily flushed outof the tubular member with drilling fluid. The composite downhole toolsare also lighter than the cast iron downhole tools and can be used inboth high and low pH environments. Details of construction of suchcomposite zonal isolation tools are found, for example, in Turley et al.U.S. Pat. No. 6,712,153, issued Mar. 30, 2004, incorporated herein byreference, and in Sutton et al., U.S. Pat. No. 6,976,534 issued Dec. 20,2005, incorporated herein by reference.

When set by a setting tool, the composite mandrel must sustain tensionin the longitudinal direction of up to about 12,000 psi, as well ascompression in the radial direction of up to about 40,000 psi. Thecomposite mandrel must also sustain internal pressure of well borefluid. Due to these forces, the fiber reinforcement of the compositematerial should have a degree of directional orientation.

The composite mandrel has been fabricated from a composite head plug anda pair of coaxial and filament-wound composite tubes. The filament-woundtubes included alternate layers of diagonal and radial fiber, forexample, diagonal layers of fiber wound criss-cross at 22 degreesinterleaved with layers of fiber wound in a circumferential wrap. Eachcomposite tube was wound on a respective steel mandrel.

The outer cylindrical surface of the inner composite tube was ground tomatch the inner diameter of the outer composite tube, so that the innercomposite tube could be closely fitted into the outer composite tube.The composite head plug was also inserted into the outer composite tube,and the composite head plug and the composite tubes were pinned andglued together. Such a composite mandrel was rather expensive due to thecost of the head plug and the cost of the two composite tubes, and thecost of grinding the inner composite tube.

SUMMARY OF THE INVENTION

It is desired to decrease the cost of fabricating a composite mandrelfor a downhole zonal isolation tool.

In accordance with one aspect, the invention provides a compositemandrel including a filament-wound composite tube, and compositematerial molded over the filament-wound composite tube.

In accordance with another aspect, the invention provides a compositemandrel for a downhole zonal isolation tool. The composite mandrelincludes a filament-wound composite tube, and composite molding sheetmaterial wound over and molded over the filament wound tube. Thecomposite molding sheet material includes chopped fibers and a matrix ofthermoset resin. The chopped fibers are arranged in layers over thefilament-wound composite tube, and the chopped fibers in each of thelayers are randomly oriented along first and second orthogonaldirections in each of the layers. The composite molding sheet materialincludes at least one sheet of the composite molding sheet materialwound over the filament-wound tube, and at least one strip of thecomposite molding sheet material wound over the sheet of the compositemolding sheet material and forming a head on the composite mandrel.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages of the invention will be describedbelow with reference to the drawings, in which:

FIG. 1 is a lateral cross-section of a bridge plug tool and a settingtool in a well casing prior to setting of the bridge plug tool;

FIG. 2 shows the bridge plug tool and the setting tool of FIG. 1 oncethe bridge plug tool has been set within the well casing;

FIG. 3 is a top view of the composite mandrel in the bridge plug tool ofFIG. 1;

FIG. 4 is a side view of the composite mandrel of FIG. 3;

FIG. 5 is a bottom view of the composite mandrel of FIG. 3;

FIG. 6 is a lateral cross-section of the composite mandrel along line6-6 in FIG. 3;

FIG. 7 is a side view of an internal plug of the composite mandrel;

FIG. 8 is a top view of an alternative construction of a compositemandrel permitting a bridge plug to be inserted into or removed from themandrel when the mandrel has been assembled into a downhole tool;

FIG. 9 is a side view of the composite mandrel of FIG. 8;

FIG. 10 is a bottom view of the composite mandrel of FIG. 8;

FIG. 11 is a lateral cross-section of the composite mandrel along line11-11 of FIG. 8;

FIG. 12 is a side view of a fiber-wound tube in the composite mandrel ofFIG. 8;

FIG. 13 is a side view of the bridge plug that may be inserted into orremoved from the mandrel when the mandrel has been assembled into adownhole tool;

FIG. 14 is a top view of the composite mandrel of FIG. 8 after thebridge plug of FIG. 13 has been inserted into the mandrel;

FIG. 15 is a lateral-cross section of the composite mandrel along line15-15 of FIG. 14;

FIG. 16 shows a first sheet of composite molding material being rolledupon an assembly of a fiber-wound tube and an internal plug and a steelcore;

FIG. 17 shows a second sheet of composite molding material being rolledupon the assembly of FIG. 16;

FIG. 18 shows a strip of composite molding material being rolled uponthe assembly of FIG. 17 to form a head upon the assembly;

FIG. 19 shows a strip of composite molding material being wrapped abovethe head of the assembly of FIG. 18;

FIG. 20 shows a final assembly resulting from the wrapping begun in FIG.19;

FIG. 21 shows the final assembly of FIG. 20 being placed into atwo-piece compression mold;

FIG. 22 shows orientations of chopped glass fiber in the compositemolding sheet material;

FIG. 23 shows a lateral cross-section of the sheets of composite moldingsheet material in the composite mandrel of FIG. 4 during the moldingprocess; and

FIG. 24 shows a lateral cross-section of the sheets of composite moldingsheet material in the composite mandrel of FIG. 11 during the moldingprocess.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown in thedrawings and will be described in detail. It should be understood,however, that it is not intended to limit the invention to theparticular forms shown, but on the contrary, the intention is to coverall modifications, equivalents, and alternatives falling within thescope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, there is shown a lateral cross-section of abridge plug tool 20 and a setting tool 21 in a well casing 22 prior tosetting of the bridge plug tool. For example, the bridge plug tool 20and the setting tool 21 are lowered by a conduit 23 into the well casing22 in order to seal a perforation 24 in the well casing 22.

The bridge plug tool 20 has an internal elongated mandrel 25 and arespective circular array of slips 26, 27 mounted on the mandrel at eachend of the bridge plug tool. Each slip has an outer surface adapted forengagement with the internal surface of the well casing 22. Each slipalso has an inclined inner surface. Each array of slips 26, 27 isdisposed next to a respective conical ring 28, 29 mounted on the mandrel25 for sliding under the inclined inner surfaces of the slips in thearray. In the middle of the sealing tool, rings 30, 31, 32 ofelastomeric sealing material are mounted on the mandrel between theconical rings 28, 29.

Once the bridge plug tool 20 has been aligned with the perforation 24,the setting tool 21 is activated. For example, the setting tool 21 has acylinder 33 and a piston 34 driven by fluid 35 under pressure, such ashydraulic fluid or gas generated by a pyrotechnic charge. The piston 34has a shaft 36 coupled by a pin 37 to a receptacle 38 for the head 40 ofthe mandrel 25 for pulling the mandrel in the longitudinal direction.

As shown in FIG. 2, when the piston 34 of the setting tool 21 pulls themandrel 25 of the bridge plug tool 20, the rings 30, 31, and 32 ofsealing material expand outward in the radial direction to seal a zoneof the well casing 22. In addition, the conical rings 28, 29 slide underthe arrays of slips 26, 27 and force the slips outward in the radialdirection into engagement with the inner wall of the well casing 22. Theslips lock the bridge plug tool 20 in place inside the well casing 22 insuch a way that the rings of sealing material 30, 31, 32 remain incompression for sealing the perforation 24 in the well casing when thesetting tool 21 is removed. For example, continued motion of the piston34 causes pins 39, 41 to shear, so that the bridge plug tool 20 becomesuncoupled from the setting tool 21. Then the conduit 23 pulls thesetting tool 21 out from the well casing 22.

If later it is desired to remove the bridge plug tool 20 from the wellcasing 22, then the bridge plug tool is drilled out destructively. Forfast drill-out, light weight, and tolerance of high and low pHenvironments, the bridge plug tool 20 is comprised of composite materialsuch as epoxy fiberglass. For example, the epoxy resin is a 50:50 blendby weight of a cycloaliphatic epoxy resin and an epoxy resin ofbisphenol A and epichlorohydrin.

The composite mandrel 40 is a relatively expensive component of thebridge plug tool 20 because the composite mandrel must sustain tensionin the longitudinal direction of up to about 12,000 psi, as well ascompression in the radial direction of up to about 40,000 psi. Thecomposite mandrel must also sustain internal pressure of well borefluid. In order to sustain these forces, the composite mandrel has beenfabricated from an outer composite tube and an inner composite tube.Each of the composite tubes has been made by a filament winding process.The filament winding process is relatively slow and requires the use ofa machine tool. Therefore, it is desired to eliminate the filamentwinding process.

It has been found that it is possible to eliminate the filament windingprocess for the outer composite tube of the composite mandrel byreplacing the outer composite tube with an over-molded compositestructure. The over-molded composite structure is fabricated fromengineered structural composite molding sheet compound by winding atleast one sheet of the composite molding sheet compound over the innercomposite tube and by fusing the layers of the composite molding sheetcompound in a molding process. The composite molding sheet compoundincludes chopped reinforcement fibers of generally constant length thatare laid down generally flat on the sheet but in random directions inthe plane of the sheet. Thus, the winding of composite molding sheetcompound over the inner composite tube results in a distribution ofreinforcement fiber orientation including hoop-wound fiber and diagonalfiber in a fashion similar to the conventional filament winding process.However, sheets and strips of the composite molding sheet compound canbe wound quickly by hand over the inner composite tube. Therefore it ispossible to reduce the cost of manufacturing the composite mandrel byabout 30 to 40 percent.

FIGS. 3, 4, 5, and 6 show that the composite mandrel 40 includes anouter tube 51 of composite molding sheet material wound over and moldedover an inner filament-wound tube 52. The composite mandrel 40 alsoincludes a head 53 formed of the over-molded composite material integralwith the outer tube 51. The molding process produces two cavities 54 and55 in the head 53. Prior to the molding process, an internal plasticplug 56 is fitted with two rubber O-rings 57 and 58 and inserted intothe head end of the inner wound composite tube 52.

FIGS. 8, 9, 10, and 11 show an alternative construction of a compositemandrel 60 permitting a bridge plug (66 in FIG. 13) to be inserted intoor removed from the mandrel 60 when the mandrel has been assembled intoa downhole tool. The composite mandrel 60 includes an outer tube 61 ofcomposite molding sheet material wound over and molded over an innerfilament-wound tube 62. The composite mandrel 60 also includes a head 63including a lower portion 69 of over-molded composite material integralwith the outer tube 61, and an upper portion 67 of filament-woundcomposite material integral with the inner tube 62. Two cavities 64 and65 are milled into the upper portion 67 of the head 63. Threads 59 areformed in a central cavity of the inner filament-wound tube 62. Forexample, after over-molding of the outer tube 61 upon the innerfilament-wound tube 62, the threads 59 are cut with a tap. The threads59 permit a bridge plug (66 in FIG. 13) to be screwed into the centralcavity of the inner filament-wound tube 62. As shown in FIG. 11, whenthe bridge plug is absent, the central cavity of the innerfilament-wound tube 62 provides a lumen for the composite mandrel topermit the flow of fluid through the down-hole zonal isolation toolincluding the mandrel.

FIG. 12 shows a side view of the inner filament-wound tube 62 andintegral upper head portion 67.

As shown in FIG. 15, after the composite mandrel 60 has been fabricated,or after the composite mandrel has been assembled into a downhole tool,the bridge plug 66 (as shown in FIG. 13) can be fitted with rubberO-rings 68, 70 and then screwed into the upper head portion 67 to plugthe inner tube 62. The bridge plug 66, for example, is made of epoxyreinforced with randomly-oriented chopped fiberglass.

Preferably the inner filament-wound tube 52 of the mandrel 40 of FIGS.3-6 and the inner filament-wound tube 62 of the mandrel 60 of FIGS. 9-11are fabricated by a filament winding process in which nine filaments arewetted with epoxy resin and then wound simultaneously under tension overa one-inch diameter steel mandrel. Each of the nine filaments includesmore than 100 glass fibers. Initially the steel mandrel is fabricated bygrinding, chrome plating, and polishing. The nine filaments are spacedover a one-half inch wide length of the mandrel during the windingprocess to form ten layers. The ten layers include alternate layers ofhoop-wound filaments and layers of criss-cross diagonal filaments at 22degrees with respect to the axis of the steel mandrel. After winding andcuring, the outer diameter of the inner filament-wound tube isapproximately 1 and ⅝ inches. Then the outer diameter of the innerfilament-wound tube is ground down to 1 and 9/16 inches, and its lengthis trimmed to 22 inches. Then a hydraulic press removes the innerfilament-wound tube from the steel mandrel.

FIGS. 16 to 21 show a method of manufacturing the mandrel 40 of FIGS.3-6 by rolling sheets and strips of engineered structural compositemolding sheet material upon the inner filament wound tube 52. Preferablythe engineered structural composite molding sheet material is LYTEX 9063(Trademark) sheet molding compound obtained from Quantum CompositesInc., 1310 South Valley Center Drive, Bay City, Mich., 48706. LYTEX 9063sheet molding compound contains 63 weight percent of 1″ chopped glassfiber and 37 weight percent of epoxy resin compound. The glass fiberdiameter is 13 microns. The epoxy resin compound is formulated withbisphenol A type epoxy resin, acid anhydride hardener and additives.

The composite molding sheet material is obtained in the form of twosheets that are 18 inches wide, 26 inches long, and 0.10 inches thick. A2 inch by 26 inch strip is cut from the end of one of these sheets, andthree 3-¼ inch by 26 inch strips are cut from the other one of thesheets. All pieces are weighted, and the total weight all pieces shouldbe 2800 grams to 2830 grams. If there is excess weight, then the extraweight is cut off the 26 inch ends of the pieces. If additional weightis needed, then a narrow strip of material is wrapped around the centerof the head after all of the pieces have been wrapped, as describedbelow.

A mold (as shown in FIG. 21) is installed in a press, and the mold ispreheated. The mold temperature is set to 300 to 310 degrees Fahrenheit.The clamp pressure on the press is set to 370 tons, and the closingspeed of the press (when slow closing starts) is set to 45 seconds.

A film of wax is put on all parts of the mold and on a steel core (71 inFIG. 16). The steel core is similar to the steel mandrel used forwinding the inner tube (52 in FIG. 16) but it has a shorter length of 22and ¼ inches. The steel core is installed into the inner fiber-woundtube with 1.6 inches of the steel core protruding from one end of theinner fiber-wound tube. At the opposite end of the inner fiber-woundtube, one drop of oil is put around the inside hole of the innerfiber-wound tube, and then the internal plug (56 in FIG. 16, with two Orings 57 and 58 installed as shown in FIG. 6) is inserted into this holeby twisting and pushing by hand. The exposed surfaces of this inner coreassembly (72 in FIG. 16) are washed with acetone, and then this innercore assembly is pre-heated to 140 to 160 degrees Fahrenheit.

For rolling the composite molding sheet material upon the inner filamentwound tube 52, the composite molding sheet material is softened byheating in an open air oven. The oven is pre-heated to 150 degreesFahrenheit. Then the sheets of the composite molding sheet material areplaced on cardboard in the oven for about 5-10 minutes or until thesheets are soft enough to roll. The sheets should not be stacked in theoven or left in the oven any longer than needed to soften them. Thesheets are taken out of the oven one piece at a time.

A first sheet (16″×26″) is taken out of the oven, and placed on analuminum plate 73 as shown in FIG. 16 so that its right side is even andaligned with the right side of the aluminum plate. Then the inner coreassembly 72 is rolled tight with the sheet 74 aligned to roll flush withthe right end of the inner core 52 and the right side even with thealuminum plate 73, as shown in FIG. 16. The steel core 71 sticks outover the right side of the aluminum plate by 1.6 inches.

A second sheet 76 (10 ¼″×26″) is taken out of the oven, and placed onthe aluminum plate 73 as shown in FIG. 16 so that its right side is evenand aligned with the right side of the aluminum plate. Then the wrappedinner core assembly 75 is placed on the second sheet 76 so that the 26″end of the first sheet 74 overlaps the 26″ end of the second sheet by ¼inch, and the wrapped inner core assembly is rolled tight with thesecond sheet aligned to roll flush with the right end of the inner coreand the right side even with the aluminum plate, as shown in FIG. 17.

A first one of the 3-¼ inch×26″ strips 78 is taken out of the oven, andplaced on the aluminum plate 73 in alignment with a pair of lines 91, 92on the aluminum plate, as shown in FIG. 18. Then the wrapped inner coreassembly 77 is placed on this first 3-¼ inch strip so that the 26″ endof the second sheet overlaps the 3-¼″ end of the first 3-14 inch stripby ¼ inch, and the wrapped inner core assembly is rolled tight with theright ends of the first and second sheets aligned even with the rightend of the aluminum plate, as shown in FIG. 17.

Then a second one of the 3-¼ inch×26″ strips is taken out of the oven,and placed on the aluminum plate in alignment with the pair of lines 91,92 on the aluminum plate 73. Then the wrapped inner core assembly isplaced on this second 3-¼ inch strip so that the 3-¼″ end of the firststrip overlaps the 3-¼″ end of the second 3-14 inch strip by ¼ inch, andthe wrapped inner core assembly is rolled tight with the right ends ofthe first and second sheets aligned even with the right end of thealuminum plate.

Then a third one of the 3-¼ inch×26″ strips 79 is taken out of the oven,and placed on the aluminum plate in alignment with the pair of lines 91,92 on the aluminum plate 73. Then the wrapped inner core assembly isplaced on this third 3-¼ inch strip so that the 3-¼″ end of the secondstrip overlaps the 3-¼″ end of the third 3-¼ inch strip by ¼ inch, andthe wrapped inner core assembly is rolled tight with the right ends ofthe first and second sheets aligned even with the right end of thealuminum plate.

At this point an upper part 81 of the wrapped inner core assembly 80 iscrimped to eliminate the cylindrical cavity formed by the winding of thefirst and second sheets 74, 75 over the inner fiber-wound tube 52 andthe internal plug 56. Then, as shown in FIG. 19, the 2″×26″ strip 82 iswrapped around the end above the head with it flaring up around the sideof the 3-¼″ wraps, in order to produce the final wrapped assembly 83shown in FIG. 20.

As shown in FIG. 21, the mold is loaded with the final wrapped assembly83 so that the head wraps are placed in the large diameter region 96 ofthe cavity and the protruding end of the steel core 71 is received inthe right end of the cavity between the pins 97, 98 on the right side ofthe mold. The final wrapped assembly 83 should be fitted so that theouter end of the 2″×26″ strip 82 is tucked into the cavity of the lowermold piece 99 and the mold pieces 99, 100 close together over the finalwrapped assembly. The composite mandrel is cured in the heated moldunder pressure for 60 minutes, and then the mold is opened slowly onejector pins. Then the steel core 71 is pulled out from the compositemandrel.

FIG. 22 shows a rectangular piece 105 of the composite molding sheetmaterial including chopped glass fibers 106, 107. Also shown are x, yand z axes aligned so that the z axis is perpendicular to the sheet ofthe composite molding sheet material. When viewed along the “z”direction, the glass fibers 106, 107 appear to be randomly oriented inthe x-y plane. When viewed along the “x” direction, the glass fibers106, 107 appear to be oriented along the “y” direction. When viewedalong the “y” direction, the glass fibers 106, 107 appear to be orientedalong the “x” direction.

FIG. 23 shows a lateral cross-section of the sheets of composite moldingsheet material in the composite mandrel 40 of FIG. 4 during the moldingprocess, before the layers of the sheets fuse together. Therefore thislateral cross-section indicates the directionality of the glassreinforcement fiber. The glass fibers are generally parallel to theplanar interfaces between adjacent layers of the sheets. Thus, there isrelatively high tensile strength in the direction of the spaced lines101, 102, 103, 104, etc., which represent the planar interfaces betweenthe adjacent layers of the sheets, so that the composite mandrel hashigh tensile strength along its length.

FIG. 24 shows a lateral cross-section of the sheets of composite moldingsheet material in the composite mandrel of FIG. 11 during the moldingprocess, before the layers of the sheets fuse together. This lateralcross-section indicates that the composite mandrel can be manufacturedas shown in FIGS. 16-18 by rolling the fiber-wound core 63 over sheetsand strips of the composite molding sheet material. In this case thesheets can be about 22.7 inches long instead of 26 inches long, andthere is no need for a 2″ wide head wrap.

In view of the above, the cost of manufacturing a composite mandrel isreduced by winding sheets or strips of composite molding sheet materialinstead of winding continuous filaments of fiber reinforcement. Thecomposite mandrel includes a filament-wound composite tube, andcomposite material molded over the filament-wound composite tube. Forexample, the composite material includes chopped fibers and a matrix ofthermoset resin. The chopped fibers are arranged in layers upon thefilament-wound composite tube, and the chopped fibers in each of thelayers are randomly oriented along first and second orthogonaldirections in each of the layers. The composite material includes atleast one sheet of the composite material wound over the filament-woundtube, and at least one strip of the composite material wound over thesheet of the composite material and forming a head on the compositemandrel. An internal cavity of the filament-wound composite tube mayprovide a lumen for the composite mandrel. In this case, the internalcavity of the filament-wound composite tube may be threaded to receive aremovable bridge plug.

What is claimed is:
 1. A composite mandrel comprising; a filament-woundcomposite tube; and composite material molded over the filament-woundcomposite tube; wherein the composite material includes chopped fibersand a matrix of thermoset resin; wherein the chopped fibers are arrangedin layers upon the filament-wound composite tube, and the chopped fibersin each of the layers are randomly oriented along first and secondorthogonal directions in said each of the layers; wherein the compositemandrel is suitable for use as a central component of a down-hole tool;and wherein the chopped fibers are parallel to planar interfaces betweenadjacent layers.
 2. The composite mandrel as claimed in claim 1, whereinthe composite material has been molded over the filament-wound compositetube by shaping the composite material in a mold.
 3. A composite mandrelcomprising; a filament-wound composite tube; and composite materialmolded over the filament-wound composite tube; wherein the compositematerial includes chopped fibers and a matrix of thermoset resin;wherein the chopped fibers are arranged in layers upon thefilament-wound composite tube and the chopped fibers in each of thelayers are randomly oriented along first and second orthogonaldirections in said each of the layers; wherein the composite mandrel issuitable for use as a central component of a down-hole tool; and whereinthe composite mandrel is capable of sustaining tension in a longitudinaldirection of 12,000 psi.
 4. The composite mandrel as claimed in claim 3,wherein the composite material has been molded over the filament-woundcomposite tube by shaping the composite material in a mold.
 5. Acomposite mandrel comprising; a filament-wound composite tube; andcomposite material molded over the filament-wound composite tube;wherein the composite material includes chopped fibers and a matrix ofthermoset resin; wherein the chopped fibers are arranged in layers uponthe filament-wound composite tube, and the chopped fibers in each of thelayers are randomly oriented along first and second orthogonaldirections in said each of the layers; wherein the composite mandrel issuitable for use as a central component of a down-hole tool; and whereinthe composite mandrel is capable of sustaining compression in a radialdirection of 40,000 psi.
 6. The composite mandrel as claimed in claim 5,wherein the composite material has been molded over the filament-woundcomposite tube by shaping the composite material in a mold.
 7. Acomposite mandrel comprising: a filament-wound composite tube; andcomposite material molded over the filament-wound composite tube;wherein the composite material includes chopped fibers and a matrix ofthermoset resin; wherein the chopped fibers are arranged in layers u son the filament-wound composite tube, and the chopped fibers in each ofthe layers are randomly oriented along first and second orthogonaldirections in said each of the layers; wherein the composite mandrel issuitable for use as a central component of a down-hole tool; and whichfurther includes a plug in one end of the filament-wound composite tube,and wherein the plug is sealed in the filament-wound composite tube bythe composite material molded over the filament-wound composite tube. 8.The composite mandrel as claimed in claim 7, wherein the compositematerial has been molded over the filament-wound composite tube byshaping the composite material in a mold.
 9. A composite mandrelcomprising: a filament-wound composite tube; and composite materialmolded over the filament-wound composite tube; wherein the compositematerial includes chopped fibers and a matrix of thermoset resin;wherein the chopped fibers are arranged in layers upon thefilament-wound composite tube, and the chopped fibers in each of thelayers are randomly oriented along first and second orthogonaldirections in said each of the layers; wherein the composite mandrel issuitable for use as a central component of a down-hole tool; and whereinthe composite material includes at least one sheet of the compositematerial wound over the filament-wound composite tube, and at least onestrip of the composite material wound over said at least one sheet ofthe composite material, said at least one strip of the compositematerial forming a head on the composite mandrel.
 10. The compositemandrel as claimed in claim 9, wherein the composite material has beenmolded over the filament-wound composite tube by shaping the compositematerial in a mold.
 11. A composite mandrel comprising: a filament-woundcomposite tube; and composite molding sheet material wound over andmolded over the filament wound composite tube; wherein the compositemolding sheet material has been molded over the filament-wound compositetube by shaping the composite molding sheet material in a mold; andwherein the composite molding sheet material includes chopped fibers anda matrix of thermoset resin, the chopped fibers are arranged in layersover the filament-wound composite tube, and the chopped fibers in eachof the layers are randomly oriented along first and second orthogonaldirections in said each of the layers; wherein the composite moldingsheet material includes at least one sheet of the composite moldingsheet material wound over the filament-wound composite tube, and atleast one strip of the composite molding sheet material wound over saidat least one sheet of the composite molding sheet material and forming ahead on the composite mandrel; and wherein the composite mandrel issuitable for use as a central component of a down-hole tool.
 12. Thecomposite mandrel as claimed in claim 11, wherein the chopped fibers areof constant length.
 13. The composite mandrel as claimed in claim 11,wherein the composite mandrel is capable of sustaining tension in alongitudinal direction of up to 12,000 psi.
 14. The composite mandrel asclaimed in claim 11, wherein the composite mandrel is capable ofsustaining compression in a radial direction of up to 40,000 psi. 15.The composite mandrel as claimed in claim 11, wherein the filament-woundcomposite tube provides a lumen for the composite mandrel.
 16. Thecomposite mandrel as claimed in claim 15, wherein an internal cavity ofthe filament-wound composite tube is threaded to receive a threaded plugfor plugging the lumen of the composite mandrel.
 17. The compositemandrel as claimed in claim 11, which includes a plug in one end of thefilament-wound composite tube, and wherein the plug is sealed in thefilament-wound composite tube by the composite molding sheet materialwound over and molded over the filament-wound composite tube.
 18. Acomposite mandrel comprising: a filament-wound composite tube; andcomposite molding sheet material wound over and molded over the filamentwound composite tube; wherein the composite molding sheet material hasbeen molded over the filament-wound composite tube by shaping thecomposite molding sheet material in a mold; and wherein the compositemolding sheet material includes chopped fibers and a matrix of thermosetresin, the chopped fibers are arranged in layers over the filament-woundcomposite tube, and the chopped fibers in each of the layers arerandomly oriented along first and second orthogonal directions in saideach of the layers; wherein the composite molding sheet materialincludes at least one sheet of the composite molding sheet materialwound over the filament-wound composite tube, and at least one strip ofthe composite molding sheet material wound over said at least one sheetof the composite molding sheet material and forming a head on thecomposite mandrel; wherein the composite mandrel is suitable for use asa central component of a down-hole zonal isolation tool; wherein thecomposite mandrel is capable of sustaining tension in a longitudinaldirection of 12,000 psi; and wherein the mandrel is capable ofsustaining compression in a radial direction of 40,000 psi.
 19. Thecomposite mandrel as claimed in claim 18, wherein the filament-woundcomposite tube provides a lumen for the composite mandrel.
 20. Thecomposite mandrel as claimed in claim 18, wherein an internal cavity ofthe filament-wound composite tube is threaded to receive a threaded plugfor plugging the lumen of the composite mandrel.
 21. The compositemandrel as claimed in claim 18, which includes a plug in one end of thefilament-wound composite tube, and wherein the plug is sealed in thefilament-wound composite tube by the composite molding sheet materialwound over and molded over the filament-wound composite tube.